The invention relates to an electric discharge tube comprising a glass envelope having at least one leadthrough which electrically connects a first electrically conductive layer provided on the inner wall of the envelope to a second electrically conductive layer provided on the outer wall of the envelope, said leadthrough consisting of an aperture which is provided in the envelope, the wall of which is coated with a third electrically conductive layer. The aperture is sealed hermetically by means of a glass plug.
The invention furthermore relates to a method of manufacturing such an electric leadthrough.
An electric discharge tube of the kind described above is disclosed in U.S. Pat. No. 2,219,107. In this patent the aperture for the leadthrough is obtained by perforating the glass envelope of the tube by means of a heated tungsten wire. In such a method, deformation of the glass envelope at the area of the aperture cannot be avoided. After having covered the wall of the aperture and the adjoining inner wall of the glass envelope with a metal layer, the aperture is sealed hermetically by means of a glass plug. This is done by heating the end of a glass rod to above its softening temperature and pressing the end in the aperture of the leadthrough. At the same time the glass envelope itself heated to near its softening temperature. As a result, again, deformation of the glass of the envelope in the proximity of the leadthrough can hardly be avoided.
Such a method is hence unfit for the manufacture of an electric leadthrough in the face plate of a camera tube because high optical quality is required of the face plate of such a tube. Deformations in the glass envelope are also undesirable in cathode ray tubes having a glass envelope of a small inside diameter, because such deformations may cause a local disturbance of the electric field distribution in the tube and may hence exert an undesired influence on the paths of the electrons in the tube. Furthermore, the known method is rather cumbersome and labor-intensive so that it is hardly suitable for use in mass production processes.